tinny sound

I have a question. When i play my bass and I try to simplify some basslines that go above fret 5 or so on one string and I bring them to the higher string, it doesn't sound near the same. I know how to mtach the frets up and all, but the sound of my D and G strings sounds extremely tinny. That's about the only way to describe it. An example is playing RHCP's Readymade on 0 and 2 on the D string instead of 5 and 7 on A. Would buying different strings help? I tried messing with the EQ on my amp (peavey TKO 80) but it didn't seem to make too much of a difference.